The molecular scale mechanism of deposition ice nucleation on silver iodide

Heterogeneous ice nucleation is a ubiquitous process in the natural and built environment. Deposition ice nucleation, i.e. heterogeneous ice nucleation that – according to the traditional view – occurs in a subsaturated water vapor environment and in the absence of supercooled water on the solid, ice-forming surface, is among the most important ice formation processes in high-altitude cirrus and mixed-phase clouds. Despite its importance, very little is known about the mechanism of deposition ice nucleation at the microscopic level. This study puts forward an adsorption-based mechanism for deposition ice nucleation through results from a combination of atomistic simulations, experiments and theoretical modelling. One of the most potent laboratory surrogates of ice nucleating particles, silver iodide, is used as a substrate for the simulations. We find that water initially adsorbs in clusters which merge and grow over time to form layers of supercooled water. Ice nucleation on silver iodide requires at minimum the adsorption of 4 molecular layers of water. Guided by the simulations we propose the following fundamental freezing steps: (1) Water molecules adsorb on the surface, forming nanodroplets. (2) The supercooled water nanodroplets merge into a continuous multilayer when they grow to about 3 molecular layers thick. (3) The layer continues to grow until the critical thickness for freezing is reached. (4) The critical ice cluster continues to grow.

Table S1: Number of ice-like structures, the total number of water molecules and the ice-like structure fraction are reported for the first hydration layer (L1) on AgI (0001) surface for the system with 5 Å water slab thickness.L, I, and W denote the hydration layer, the number of ice, and the number of water, respectively.S6: Number of ice-like structures, the total number of water molecules and the ice-like structure fraction are reported for the first (L1), second (L2), third (L3), fourth (L4), Fifth (L5) and sixth (L6) hydration layers on AgI (0001) surface for the system with 30 Å water slab thickness.L, I, and W denote the hydration layer, the number of ice, and the number of water, respectively.
We performed four parallel GCMC/MD simulations of 6 kPa vapor pressure at 253 K.
The number of water molecules in the first adsorbed layer is higher than in the second, third and fourth layers.However, the number of water molecules in the first layer was relatively low, and the highest count we observed in this layer over a period of 600 ps was 35 (as can be seen in Figure S1).We estimated to see the convergence to the mentioned target pressure, a substantial amount of time was required due to the limited parallelizability of GCMC/MD simulations.Therefore, we did not continue the simulation for the vapor pressure of 6 kPa.
Figure S1: The temporal evolution of water adsorption on AgI (0001).The adsorbate is segmented into layers, each having a width of 5 Å.The vapor uptake is performed at 253K with 6 kPa.
At a vapor pressure of 60 kPa, four separate simulations were performed for each MC/MD ratio of 500 to 100 and 500 to 200, all conducted at a temperature of 253 K. Figure S2 shows the fluctuation in entire simulation trajectories in all four realizations.The highest amount of water molecules in this ratio was about 460 water molecules within 10 ns.We did not observe any stable adsorbed layer for this ratio.Figure S3 illustrates the ratio of MC/MD 2.5 exhibiting similar behavior to the ratio of 2.2.However, in these four parallel simulations, there was a slight delay in the average convergence time to the target pressure compared to the ratio of 2.2.The number of ice in this ratio is also identified and the results indicate an upward trend in two out of four realizations as can be seen in Figure S4.

Figure S2 :
Figure S2: Temporal evolution of water adsorption on AgI (0001) in 4 parellel simulations.The adsorbate is segmented into layers, each having a thickness of 5 Å.The vapor uptake is simulated at 253 K with P= 60 kPa.

Figure S3 :
Figure S3: Temporal evolution of water adsorption on AgI (0001) in 4 parellel simulations.The adsorbate is segmented into layers, each having a thickness of 5 Å.The vapor uptake is simulated at 253 K with P= 60 kPa.

Table S2 :
Number of ice-like structures, the total number of water molecules and the icelike structure fraction are reported for the first (L1) and second hydration layer (L2) on AgI (0001) surface for the system with 10 Å water slab thickness.L, I, and W denote the hydration layer, the number of ice, and the number of water, respectively.

Table S4 :
Number of ice-like structures, the total number of water molecules and the ice-like structure fraction are reported for the first (L1), second (L2), third (L3) and fourth (L4) hydration layers on AgI (0001) surface for the system with 20 Å water slab thickness.L, I, and W denote the hydration layer, the number of ice, and the number of water, respectively.

Table S5 :
Number of ice-like structures, the total number of water molecules and the icelike structure fraction are reported for the first (L1), second (L2), third (L3), fourth (L4) and Fifth (L5) hydration layers on AgI (0001) surface for the system with 25 Å water slab thickness.L, I, and W denote the hydration layer, the number of ice, and the number of water, respectively.